Drew is pretty much right on target. There are a lot of transducers (sensors that provide an electrical output) that use magnetism in some way. For instance, the reluctors for both the cam and crank in an LS series engine uses the absence or presence of ferrous material (iron or steel) to modulate a magnetic field that is sensed and sent to the PCM in the form of voltage/current changes. The notches in the reluctor wheel indicating rotational position of the crank are exactly mirrored by an electrical pulse train to the PCM to determine RPM, spark and injector timing, sustaining fuel pump operation and other things.

Back to the Hall Effect Device question. The most common specific use of these devices is to measure the current through a conductor. This could be for a clamp on ammeter like an electrician uses, or to trigger a nearly instantaneous circuit protection device (similar to a fuse or circuit breaker). There are lots of inductive (a coil of wire) devices that sense magnetic fields produced by an electrical current (like a clamp on spark sensor for a timing light). This type of device is much less expensive than than a Hall Effect Sensor and far more common.

The reason I ask is because im running a Tec3 computer and we're trying to figure out what sensor a Porsche 928 would be running for a cam sensor. The Tec3 gives you the option to run either a magnetic or hall effect sensor

If I were to guess on a cam sensor its probably hall effect. Magnetic pickups suck IMO. I've dealt with a datsun that was using magnetic pickups for crank angle. Radiator fan worked its way loose and smacked the crank pulley killing the magnets. You have no idea how hard it is to dig rare earth magnets out of a crank pulley on the side of a road so you can glue some back in......

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The main difference in the 2 are the signals they produce.
A magnetic pickup produces an alternating current waveform. The faster the magnets pass the pickup, the higher the frequency and the amplitude.
A hall effect switch uses a 3 wire pickup (+/-/signal) and produces a square wave DC signal.
A hall effect switch is much more precise. The signal is much cleaner, and doesnt have to be converted. It can be used by the PCM in its raw form. An alternating current signal has to be converted at some point. Sometimes using an external buffer, sometimes the conversion is done inside the module (ECM/PCM/VCM/EBCM,etc)
The easiest way to tell what you have is to scope it.

Hey ed, I don't mean to question what you're saying, but I was fairly sure hall effect sensors are also sine wave producers. IIRC the only thing that produces a square wave is an optical pickup. I have the understanding that as the "fingers" of the hall effect wheel approach the sensor it will start to produce the sine wave peaking when the center of the "finger" passes over the sensor, then falling back down. Guess I need to find one and hook it up to an o-scope up here at work.

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"Frequently, a Hall sensor is combined with circuitry that allows the device to act in a digital (on/off) mode, and may be called a switch in this configuration."

Circuitry should be a capacitor or two and an SCR/transistor. Hall sensors are definately sine wave-form output.

Interesting. You scope a Ford hall effect sensor in a distributor, and its a square wave. Must be integrated electronics that convert it.
Look at cam and crank signals in the LS motors. Square wave.
Late model speed sensors. Again square wave. They must be integrating all of them with the required electronics (ICs) to convert the signals. I know they are in the later model speed sensors.

Just out of boredom, when I get to work today I'll use an o-scope on a hall effect sensor that I know doesn't have the required stuff to turn it into a square wave and take a pic. Its not exactly automotive application but a pin detection system that works at 40+Hz and counts every pin as it passes the sensor to make sure there's no mechanical issues on a conveyor belt. Just like a standard hall effect sensor just moving in a straight line instead of a circle where measured.

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Interesting. You scope a Ford hall effect sensor in a distributor, and its a square wave. Must be integrated electronics that convert it.
Look at cam and crank signals in the LS motors. Square wave.
Late model speed sensors. Again square wave. They must be integrating all of them with the required electronics (ICs) to convert the signals. I know they are in the later model speed sensors.

Lots of ford sensors have an anolog to digital converter inside them.
Thats why you get a digital square wave when scoping them.
Most times you can ID if it is a magnetic pulse generating sensor by 2 wires.
The hall effect is externally powered & has 3 wires.
But not always a rule of thumb, as sometimes an extra wire is added as a sheild agaist RFI.

A magnetic pick up will use two wires, with either a rotating metal component inducing an a.c. voltage (sine wave) in a fixed coil of wire surrounding a magnet (basic electronic ignition pick up) or a moving magnet inducing a voltage in a fixed coil of wire (think of an MSD crank trigger).
A hall effect (three wire) usually uses a blade to change a magnetic field, causing an A.C. voltage which triggers a transistor. This transistor will toggle a supplied voltage (usually 5v on automotive computers) to go from 0v to 5v in a square wave output. A digital (square) signal is more precise because its either on or off, no guesswork.
Hope I confused the heck out of the issue.
I hope what I said is right anyway...
Johnny

Spot on Johnny, that 5v signal is actually pretty standard on 75% of electronic controls devices. OEM's prefer to have transistors and whatnot in the sensors themselves now so if a sensor goes bad or you loose a transistor it no longer means you need an ecu or to have the ecu repaired, its just one sensor away.

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I'm with edcmat-l1. Every hall effect sensor I've worked with in an automotive application was an on/off configuration that produced a square wave signal when scoped. For example, it'll put out a +5V signal when on, 0V when off.